Windows 7 only: System volume mixer replacement SndVolPlus does everything the default system mixer does, with one addition: middle-clicking the system tray icon mutes the volume.

Once you've downloaded the application and added it to your startup folder, you'll need to disable the default mixer icon through the Taskbar Properties -> Notification Icons -> System Icons screen. The new mixer icon will work the same way as the old one, except you can now middle-click the icon to mute the system volume instantly.